When a ligament becomes detached from a bone, surgery usually is required to reconstruct the ligament. Often, a substitute ligament or graft is secured into bone tunnels to facilitate the incorporation of the ligament and permanent attachment.
An example of this type of surgery is the reconstruction of torn knee ligaments, particularly the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). Surgical reconstruction is the standard of care after cruciate ligament injuries. Tears of knee ligaments can render the knee unstable leading to recurrent episodes of giving way. Reconstruction of these ligaments by using tendon grafts can result in restoration of knee stability and function. To perform a ligament reconstruction, remnants of the torn ligament are removed. Next, bone tunnels are drilled in the femur and tibia bones at the attachment site for the ligament to be reconstructed. A tissue graft is then spanned between the tunnels with the graft ends buried in the bone tunnels. Finally, the graft is tensioned and attached securely to the bone tunnels. The graft then undergoes a slow process of healing, which creates a firm attachment of the graft to bone, and establishes a new blood supply for the graft. Over a period of 6-18 months, the graft remodels to become living tissue, which can resist abnormal motions of the knee that would result in giving way.
The fixation of the graft to the bones is of paramount importance in this type of surgeries as any loosening of the graft can result in failure to restore knee stability. Furthermore, early knee motion and exercises which help the patient to recover quickly from surgery can place significant stress on the graft fixation. Adequate stability of graft fixation must be achieved to allow the patient to safely benefit from the effects of early rehabilitation.
Several types of tissue grafts are available for use in knee ligament reconstruction. Each type of graft has certain advantages and disadvantages. The use of autologous hamstring tendons in knee ligament reconstruction has grown in popularity because this graft causes very minimal morbidity to harvest, does not disrupt the extensor mechanism, creates a very strong soft tissue graft and does not expose a patient to the risks of using cadaver tissue. However, fixation of hamstring grafts to the proximal tibial bone tunnel remains a weak link in successful use of the hamstring tendons. The proximal tibia often contains soft bone providing a weak substrate for implant fixation. A second problem is that the tibia is only covered by a thin layer of soft tissue and a prominent implant can often be palpated beneath the skin causing pain. Other types of soft tissue grafts, both of autologous and cadaver tissue, including achilles tendon, quadriceps tendon, fascia lata and palmaris longus tendon are used in certain situations to reconstruct the ligaments. Again the weak link of fixation to the bone tunnel remains a problem.
A number of devices are known in the prior art for fixation of soft tissue grafts to bones. However, as it will become evident, each of these devices possesses problems that limit their successful use.
The first class of devices for soft tissue to bone fixation is suture anchor, such as the anchor illustrated by U.S. Pat. No. 5,472,452. In his description, Trott discloses a bone anchor which can be placed in a small bone hole, and then serves as the attachment site for soft tissues which are sutured to the bone. Similar type devices are further disclosed by Lee and Sander in U.S. Pat. No. 5,480,403 and by Hayhurst in U.S. Pat. No. 5,601,557. A variation on this device is disclosed by Ross, Snyder, Marchand in U.S. Pat. No. 5,246,441 where a tack is used to secure soft tissues against the bone as it is driven into a small bone hole. These devices do not provide sufficient holding strength suitable for fixation of knee ligament grafts.
A second class of fixation devices is illustrated by U.S. Pat. Nos. 4,454,875 and 4,570,623 where metal staples with spikes on the underside are seen securing ligament tissue to bone. These devices are undesirable in that they have less than ideal fixation strength. The devices are often difficult to use and reposition may cause damage to the bone. They may also be palpable beneath the skin causing pain and necessitating a second operation for their removal.
A third class of fixation device is the interference screw. An example of this type of implant is the RCI® screw marketed by Smith-Nephew Endoscopy Corp (Boston, Mass.). The RCI® screw has blunt threads to avoid damage to a soft tissue graft. This screw is threaded into a bone tunnel alongside of the strands of a soft tissue graft. The screw holds the graft in place by compressing the soft tissue graft against the sides of the bone tunnel. Unfortunately, interference screws have poor fixation strength against soft tissue grafts.
A forth class of fixation device uses a screw-washer combination design. This device consists of a standard bone screw and a washer, which is placed distal to the bone tunnel. The screw may be tightened down over a soft tissue graft capturing it against the tibia or can act as an anchor around which to tie sutures. U.S. Pat. No. 6,123,711 by Winters et al. discloses such a fixation system with the screw-washer design. However, the screw-washer assembly is often prominent, causing pain and requiring a second operation for implant removal. The tack described by Winters et al. is only inserted into the top portion of the outside screw, resulting in weak graft fixation leaving concerns of graft slippage. In addition, the tack is retained onto the screw by the barb head of the tack, surgeons are not able to adjust the tightness of compression exerted by the washer.
A fifth class of fixation device is a button over which sutures may be tied. Fixation strength of this type of device is limited by the strength of the sutures. In some cases the button may be too prominent and requires post-surgery removal.
Other implants such as the Endobutton® of Smith-Nephew Endoscopy Corp. (Boston, Mass.) and various types of threaded pins over which the graft is wrapped are only applicable to fixation of the tendon graft on the femoral side of an ACL reconstruction and thus do not provide a solution to the weak link on the tibial side.
The disadvantages of the available methods of soft tissue fixation can be summarized as:                A. poor fixation strength allowing slippage of the graft during early rehabilitation;        B. limited bone to tendon interface for healing;        C. prominence of the implant which may cause pain;        D. difficult to adjust fixation;        E. requirement of second surgery for implant removal;        F. damage to bone by implant;        G. implants not amenable to tibia sided graft fixation.        